Monroeville police were called to Cambridge Square Apartments around 9 p.m. on Friday, where the toddler was found with a single gunshot wound to the head, authorities said.
The county medical examiner's office ruled the death a homicide.
"There is strong evidence that [another child] accidentally shot the victim while handling an unsecured firearm inside the residence," according to a statement.
The statement goes on to say the weapon was legally owned by an adult who is cooperating with the investigation.
The county prosecutor’s office has requested all details of the investigation be released “for determination of culpability.”
The Allegheny County children, youth and families agency has been informed of the death, according to police.
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